Stability is the Key

Aug 01, 2004

Biopharmaceutical formulation is primarily about stability and the product's mode of use. The route of administration (drug delivery, see and the manufacturing process must be considered, along with the molecule's intrinsic stability factors. Because proteins and peptides are such large molecules and exist to interact with their environment, they are somewhat fragile. They must be protected from denaturation and degradation until they can be delivered to their site of action in a patient's body.

The biopharmaceutical development process does not allow enough time to confirm stability requirements for a final formulation (which could take two years) before the company is otherwise ready to apply to market the product. Initial indications should be developed during clinical studies, so formulators must begin with three-, six-, and nine-month tests of their molecule's structure and innate stability using various analytical methods. "Accelerated" stability tests subject products to various stresses: a range of pH values, heat, light, freezing and thawing conditions, additives, and surface materials and interfaces. The test for agitation-induced denaturation is performed by swirling (creating a vortex inside the vials), rotating vials at elevated temperatures, and testing the surface tension of the liquid formulation.

"The experiments necessary to define an acceptable formulation depend on the proposed route of administration," said D.B. Lakings, principal consultant at Drug Safety Evaluation Consulting. Will the drug be injected, inhaled, or delivered some other way? Consider stability in the bloodstream, Lakings suggests. A poorly soluble drug could precipitate out of solution in vivo — not only ruining its chance of doing its job but possibly endangering the patient. "Formulation excipients can possibly be added to prevent degradation," he said. Or the formulation may need to be stored under certain conditions — refrigerated, frozen, freeze dried, or kept in reduced-light yellow or brown vials, for example. The less stringent the storage conditions, the better.

The current market demands one to two years of storage for most products. During preclinical testing, "Samples obtained before shipment (and stored under conditions known to provide stability) and after shipment to a testing laboratory should be analyzed to provide information on potential problems in shipping," Lakings said. A protein that degrades in less than six months can be a problem if stability claims are declared after only three months of testing. Of course, it's normal to want to rush the development process in order to get a product to the market as soon as possible. After three months, a stability protocol and results can be sent to CBER, but it is good business practice to amend that information approximately every three months, extending the product's expiration date as far as possible. Considerations for making formulation decisions follow this order: (1) shelf life, (2) convenience of use, and (3) cost-effectiveness.

What Can Go Wrong Here's one of the scariest things that can happen at a biopharmaceutical plant: A quality inspector looks at vials of final, liquid-formulated product and sees globs of brown gunk floating or settling to the bottom. An entire batch, worth at least thousands of dollars, has just been wasted. Usually the failure is not so obvious. Formulation scientists work hard to make sure aggregation and precipitation don't happen.

Biopharmaceutical formulators must evaluate protein formulations for degradation products both qualitatively and quantitatively. Qualitative studies include preclinical safety studies and tests that determine how degradation might be prevented by optimizing the formulation. Quantitative evaluation examines temperature dependencies, for example, or a formulation's tendency to convert in vivo to native forms (if it has denatured). Bioassays study protein activity, identity, and critical pathways. Analytical methods are developed in parallel with the product itself, with at least two solid techniques chosen to assess stability.

Chemical degradation changes the primary structure of a protein. Bond cleavage will create an entirely new molecule. Such chemical degradation is usually preceded by a causal physical process, typically unfolding, that makes available residues that are usually inaccessible for chemical reactions with their environment. Physical degradation changes only the higher-order structure (secondary, tertiary, quaternary) of the polypeptide, not necessarily creating a brand new molecule. Such degradation includes aggregation, adsorption, unfolding, and precipitation.

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